A hot spring, thermal spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow bodies of magma (molten rock) or by circulation through faults to hot rock deep in the Earth's crust.
Hot spring water often contains large amounts of dissolved minerals. The chemistry of hot springs ranges from acid sulfate springs with a pH as low as 0.8, to alkaline chloride springs saturated with silica, to bicarbonate springs saturated with carbon dioxide and carbonate minerals. Some springs also contain abundant dissolved iron. The minerals brought to the surface in hot springs often feed communities of extremophiles, microorganisms adapted to extreme conditions, and it is possible that life on Earth had its origin in hot springs.
Humans have made use of hot springs for bathing, relaxation, or medical therapy for thousands of years. However, some are hot enough that immersion can be harmful, leading to scalding and potentially death.
Definitions
There is no universally accepted definition of a hot spring. For example, one can find the phrase hot spring defined as any spring heated by geothermal activity a spring with water temperatures above its surroundingsMiriam-Webster Online dictionary definition of hot springColumbia Encyclopedia, sixth edition, article on hot spring a natural spring with water temperature above human body temperature (normally about )Wordsmyth definition of hot springAmerican Heritage dictionary, fourth edition (2000) definition of hot spring provides a critical discussion of the definition of a hot spring.Infoplease definition of hot springRandom House Unabridged Dictionary, © Random House, Inc. 2006. definition of hot spring
a natural spring of water whose temperature is greater than Wordnet 2.0 definition of hot springUltralingua Online Dictionary definition of hot springRhymezone definition of hot springLookwayup definition of hot spring a type of thermal spring whose water temperature is usually or more above mean air temperature. Leet states that there are two types of thermal springs; hot springs and warm springs. Note that by this definition, "thermal spring" is not synonymous with the term "hot spring". a spring with water temperatures above US NOAA Geophysical Data Center definition
The related term "warm spring" is defined as a spring with water temperature less than a hot spring by many sources, although Pentecost et al. (2003) suggest that the phrase "warm spring" is not useful and should be avoided. In 1923, Menzier proposed that a warm spring be defined as a thermal spring where the water is below that of the human body, but above that of the mean air temperature around the spring, though this definition is contested.
Sources of heat Water issuing from a hot spring is heated geothermally, that is, with heat produced from the Earth's mantle. This takes place in two ways. In areas of high volcanic activity, magma (molten rock) may be present at shallow depths in the Earth's crust. Groundwater is heated by these shallow magma bodies and rises to the surface to emerge at a hot spring. However, even in areas that do not experience volcanic activity, the temperature of rocks within the earth increases with depth. The rate of temperature increase with depth is known as the geothermal gradient. If water percolates deeply enough into the crust, it will be heated as it comes into contact with hot rock. This generally takes place along faults, where shattered rock beds provide easy paths for water to circulate to greater depths.
Much of the heat is created by decay of naturally radioactive elements. An estimated 45 to 90 percent of the heat escaping from the Earth originates from radioactive decay of elements mainly located in the mantle. The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232. In areas with no volcanic activity, this heat flows through the crust by a slow process of thermal conduction, but in volcanic areas, the heat is carried to the surface more rapidly by bodies of magma.
A hot spring that periodically jets water and steam is called a geyser. In active volcanic zones such as Yellowstone National Park, magma may be present at shallow depths. If a hot spring is connected to a large natural cistern close to such a magma body, the magma may superheat the water in the cistern, raising its temperature above the normal boiling point. The water will not immediately boil, because the weight of the water column above the cistern pressurizes the cistern and suppresses boiling. However, as the superheated water expands, some of the water will emerge at the surface, reducing pressure in the cistern. This allows some of the water in the cistern to flash into steam, which forces more water out of the hot spring. This leads to a runaway condition in which a sizable amount of water and steam are forcibly ejected from the hot spring as the cistern is emptied. The cistern then refills with cooler water, and the cycle repeats.
Geysers require both a natural cistern and an abundant source of cooler water to refill the cistern after each eruption of the geyser. If the water supply is less abundant, so that the water is boiled as fast as it can accumulate and only reaches the surface in the form of steam, the result is a fumarole. If the water is mixed with mud and clay, the result is a mud pot.
An example of a non-volcanic warm spring is Warm Springs, Georgia (frequented for its therapeutic effects by paraplegic U.S. President Franklin D. Roosevelt, who built the Little White House there). Here the groundwater originates as rain and snow (meteoric water) falling on the nearby mountains, which penetrates a particular formation (Hollis Quartzite) to a depth of and is heated by the normal geothermal gradient.
Chemistry
Because heated water can hold more dissolved solids than cold water, the water that issues from hot springs often has a very high mineral content, containing everything from calcium to lithium and even radium. The overall chemistry of hot springs varies from alkaline chloride to acid sulfate to bicarbonate to iron-rich, each of which defines an end member of a range of possible hot spring chemistries.
Alkaline chloride hot springs are fed by hydrothermal fluids that form when groundwater containing dissolved chloride salts reacts with silicate rocks at high temperature. These springs have nearly neutral pH but are saturated with silica (). The solubility of silica depends strongly upon temperature, so upon cooling, the silica is deposited as geyserite, a form of opal (opal-A: ). This process is slow enough that geyserite is not all deposited immediately around the vent, but tends to build up a low, broad platform for some distance around the spring opening.
Acid sulfate hot springs are fed by hydrothermal fluids rich in hydrogen sulfide (), which is oxidized to form sulfuric acid, . The pH of the fluids is thereby lowered to values as low as 0.8. The acid reacts with rock to alter it to clay minerals, oxide minerals, and a residue of silica.
Bicarbonate hot springs are fed by hydrothermal fluids that form when carbon dioxide () and groundwater react with carbonate rocks. When the fluids reach the surface, is rapidly lost and carbonate minerals precipitate as travertine, so that bicarbonate hot springs tend to form high-relief structures around their openings.
Iron-rich springs are characterized by the presence of microbial communities that produce clumps of oxidized iron from iron in the hydrothermal fluids feeding the spring.
Some hot springs produce fluids that are intermediate in chemistry between these extremes. For example, mixed acid-sulfate-chloride hot springs are intermediate between acid sulfate and alkaline chloride springs and may form by mixing of acid sulfate and alkaline chloride fluids. They deposit geyserite, but in smaller quantities than alkaline chloride springs.
Flow rates
Hot springs range in flow rate from the tiniest "seeps" to veritable rivers of hot water. Sometimes there is enough pressure that the water shoots upward in a geyser, or fountain.
High-flow hot springs There are many claims in the literature about the flow rates of hot springs. There are many more high flow non-thermal springs than geothermal springs. Springs with high flow rates include: The Dalhousie Springs complex in Australia had a peak total flow of more than 23,000 liters/second in 1915, giving the average spring in the complex an output of more than 325 liters/second. This has been reduced now to a peak total flow of 17,370 liters/second so the average spring has a peak output of about 250 liters/second. The 2,850 hot springs of Beppu in Japan are the highest flow hot spring complex in Japan. Together the Beppu hot springs produce about 1,592 liters/second, or corresponding to an average hot spring flow of 0.56 liters/second. The 303 hot springs of Kokonoe in Japan produce 1,028 liters/second, which gives the average hot spring a flow of 3.39 liters/second. Ōita Prefecture has 4,762 hot springs, with a total flow of 4,437 liters/second, so the average hot spring flow is 0.93 liters/second. The highest flow rate hot spring in Japan is the Tamagawa Hot Spring in Akita Prefecture, which has a flow rate of 150 liters/second. The Tamagawa Hot Spring feeds a wide stream with a temperature of . The most famous hot springs of Brazil's Caldas Novas ("New Hot Springs" in Portuguese) are tapped by 86 wells, from which 333 liters/second are pumped for 14 hours per day. This corresponds to a peak average flow rate of 3.89 liters/second per well. In Florida, there are 33 recognized "magnitude one springs" (having a flow in excess of ). Silver Springs, Florida has a flow of more than . The Excelsior Geyser Crater in Yellowstone National Park yields about . Evans Plunge in Hot Springs, South Dakota has a flow rate of of spring water. The Plunge, built in 1890, is the world's largest natural warm water indoor swimming pool. The hot spring of Saturnia, Italy with around 500 liters a secondTerme di Saturnia , website Lava Hot Springs in Idaho has a flow of 130 liters/second. Glenwood Springs in Colorado has a flow of 143 liters/second. Elizabeth Springs in western Queensland, Australia might have had a flow of 158 liters/second in the late 19th century, but now has a flow of about 5 liters/second. Deildartunguhver in Iceland has a flow of 180 liters/second. There are at least three hot springs in the Nage region south west of Bajawa in Indonesia that collectively produce more than 453.6 liters/second. There are another three large hot springs (Mengeruda, Wae Bana and Piga) north east of Bajawa, Indonesia that together produce more than 450 liters/second of hot water.
Ecosystems
Hot springs often host communities of microorganisms adapted to life in hot, mineral-laden water. These include thermophiles, which are a type of extremophile that thrives at high temperatures, between . Further from the vent, where the water has had time to cool and precipitate part of its mineral load, conditions favor organisms adapted to less extreme conditions. This produces a succession of microbial communities as one moves away from the vent, which in some respects resembles the successive stages in the evolution of early life.